Organic light-emitting diodes (OLED) generate light by re-combination of electrons and holes, and emit light when a bias is applied between the anode and cathode such that an electrical current passes between them. The brightness of the light is related to the amount of the current. If there is no current, there will be no light emission, so OLED technology is a type of technology capable of absolute blacks and achieving almost “infinite” contrast ratio between pixels when used in display applications.
Several approaches are taught in the prior art for pixel thin film transistor (TFT) circuits to deliver current to an element of a display device, such as for example an organic light-emitting diode (OLED), through a drive transistor. In one example, an input signal, such as a high “SCAN” signal, is employed to a switch transistor in the circuit to permit a data voltage, VDAT, to be stored at a storage capacitor during a programming phase. When the SCAN signal is low and the switch transistor isolates the circuit from the data voltage, the VDAT voltage is retained by the capacitor and this voltage is applied to a gate of a drive transistor. With the drive transistor having a threshold voltage VTH, the amount of current to the OLED is related to the voltage on the gate of the drive transistor by:
      I          O      ⁢      L      ⁢      E      ⁢      D        =            β      2        ⁢                  (                              V                          D              ⁢              A              ⁢              T                                -                      V                          O              ⁢              L              ⁢              E              ⁢              D                                -                      V                          T              ⁢              H                                      )            2      
TFT device characteristics, especially the TFT threshold voltage VTH, may vary, for example due to manufacturing processes and/or stress and aging of the TFT device during the operation. With the same VDAT voltage, the amount of current delivered by the TFT drive transistor could vary by a large amount due to such threshold voltage variations. Therefore, pixels in a display may not exhibit uniform brightness for a given VDAT value. In addition, OLED device characteristics may vary due to manufacturing processes, and/or stress and aging during the operation of the OLED. For example, the threshold voltage of the OLED for light emission may change. Conventional circuit configurations, therefore, often include elements that operate to compensate for at least some of these component variations to achieve an OLED display with more uniform brightness among sub-pixels.
Conventionally, therefore, OLED pixel circuits have high tolerance ranges to variations in threshold voltage and/or carrier mobility of the drive transistor by employing circuits that compensate for mismatch in the properties of the drive transistors. For example, an approach is described in U.S. Pat. No. 7,414,599 (Chung et al., issued Aug. 19, 2008), which describes a circuit in which the drive TFT is configured to be a diode-connected device during a programming period, and a data voltage is applied to the source of the drive transistor. Such a circuit configuration, however, may not be suitable for a circuit architecture in which the OLED is driven by the source of the drive transistor, such when the drive transistor particular is an indium gallium zinc oxide (IGZO) transistor. IGZO transistors are beneficial because they experience low current leakage when in the off state. In such architecture, the OLED current is affected by the voltage at anode of the OLED, which can lead to variation in the emission.
Another circuit approach is described in U.S. Pat. No. 8,242,983 (Myoung-Hwan Yoo, issued Aug. 14, 2012). In such circuit, a capacitor is connected between the gate and source of the drive transistor with a first plate of the capacitor connected to the anode of the OLED and a second plate connected to the gate of the drive transistor. A threshold voltage is stored across the capacitor between the gate and source of the drive transistor during the threshold compensation phase. During emission, the second plate of the capacitor, to which the gate of the drive transistor is connected, is floating. Any variation at the anode of the OLED will be followed by the gate of the drive transistor. In this way, the threshold voltage of the drive transistor and the voltage variation of the OLED can be compensated. A disadvantage of this configuration is that the data voltage is difficult to store in the capacitor between the gate and the source of the drive transistor, which can reduce accuracy of the data voltage application.
Another circuit approach is described in U.S. Pat. No. 9,666,125 (Chieh-Hsing CHUNG, issued May 30, 2017). In such circuit, one capacitor is used to store the threshold voltage of the drive transistor, and another capacitor is used to store the data voltage. During emission, the two capacitors are series connected between the gate and source of the drive transistor. The threshold voltage and the data voltage are stored between the gate and source of the drive transistor. Any variation at the anode of the OLED will be followed by the gate of the drive transistor. In this way, the data is properly programmed, and the threshold voltage of the drive transistor and the voltage variations of the OLED are compensated. A disadvantage of this configuration, however, is that as the capacitors are series connected, the effective capacitance is smaller than any one capacitor. As there are leakage though the transistor switches, the gate voltage of the drive transistor can drift a large amount over one frame time, and thus the current to the OLED can vary significantly during a frame. To reduce such voltage drift, a large capacitor may be needed, but using a large capacitor may not be a viable solution for high resolution applications in which reduced space is required.
Another circuit approach is described in U.S. Pat. No. 9,196,196 (Bo-Yong Chung, issued Nov. 14, 2015). During the compensation phase, the OLED voltage and threshold voltage of the drive transistor are stored in a storage capacitor. During the data programming phase, the data is distributed between the storage capacitor and a second capacitor. In this way, the OLED voltage, threshold voltage, and data voltage are programmed to the storage capacitor. This configuration, however, has a disadvantage of resulting in a large current flowing through the OLED during the compensation phase, which could lead to light emission during the compensation phase. The light emission during the compensation phase will degrade the blackness and compromise the contrast ratio.